This invention relates to a drum brake device for a rear wheel, which comprises a single brake shoe which functions as a leading shoe with self-servo property either when the vehicle is moving forward or when it is moving backward.
Japanese Patent Application Publication No. Heisei 9-324829, illustrates a drum brake device which comprises a single brake shoe with a self-servo property when a vehicle is moving either forward or backward.
An example of this type of drum brake device is explained with reference to FIG. 12. In FIG. 12, a single brake shoe a is movably mounted on the back plate. An upper portion of the brake shoe a engages with the anchor b. A lower portion engages with a shoe clearance adjustment device c. A link d is rotatably pivoted at the central region of the brake shoe a. The upper portion of the link d functionally engages with the anchor b. The lower portion of the link d abuts against a service brake actuator e operating when the service brake is activated. The upper portion of the brake shoe a is supported by the anchor b when the brake drum rotates in one direction (the direction shown by the arrow A). The lower portion of the brake shoe a is supported by the shoe clearance adjustment device c when the brake drum rotates in the reverse direction (opposite to the arrow A).
There are two points which need to be considered when designing a drum brake device. First, a brake fluid pipe to be connected to a wheel cylinder is desirably kept a greater distance from the road surface in order to avoid damaging the pipe because of flying gravel, debris and the like.
Secondly, if the rear wheels of the vehicle lock before the front wheels, handling becomes more difficult, and the driver could be in danger. Therefore, it is important to avoid such a situation.
In addition, as shown in the conventional art, if the shoe functions as a floating shoe when the brake drum rotates in any direction, it is well-known that the braking effect can be increased as the distance from a horizontal straight line X across the center of the brake to the supporting point of the brake shoe becomes shorter. If the dimensions or size relative to the braking effect is same, it is well-known that the braking effect of a pivot shoe rather than a floating shoe is lower if the drum brake is designed as an ordinary type. The detail is described in "BRAKE DESIGN AND SAFETY" by Rudolf Limpert, SAE International, pages 61-75, and "Tire, Brake" of "Automobile Engineering Encyclopedia", Sankaido.
From the above-described view, the wheel cylinder of the drum brake device of FIG. 12 may be located at the upper portion of the device when applied to the right wheel side of the vehicle, thereby securing the braking effect of the vehicle moving backward and arranging the run of a parking brake cable to be directed toward the front of the vehicle. However, the available space for arranging the shoe clearance adjustment device supporting the brake shoe when moving forward becomes limited, thereby making it difficult to design a device to eliminate the problem of the rear wheels being locked first.